Millimeter wave (mmW) communication has been considered as an important technology to be employed for modern communication standards such as the future 5G mobile system. To mitigate the severe path loss due to very high frequency, beamforming technique becomes a crucial technique to achieve reasonable link budgets. It is envisioned that so called pencil beams with very narrow beamwidths, e.g., 5 to 15 degrees, offering high beamforming gain have to be widely used. Moreover, due to the fact that digital beamforming may be too costly or require too much power to be implemented in practical mmW system, analog beamformers comprised of networks of phase shifters and possibly variable attenuators or amplifiers are typically employed. The more beams are to be generated, the more phase shifters, combiners and possibly attenuators are required, therefore it may not be commercially feasible to provide as many beams as UEs. To transmit multiple parallel beams in parallel, mmW access point (AP) can be equipped with multiple beamformers, each of which illuminates a different beam direction. In addition to higher throughput, modern communication standards such as the future 5G mobile system are also targeting to lower latency and better spectrum efficiency.
Hence, there is a need to improve beamforming with respect to high beamforming gain, low latency and high spectrum efficiency. In particular for cases where there are more UEs than available beams reducing latency is a critical issue.